The proposed work is intended to study the mechanism of action of the biologically important and other structurally related synthetic Vitamin D3 metabolites on bone, in vitro. Using isolated, cultured bone cells, Vitamin D3 metabolites will be tested on 45 Ca-uptake and Cyclic-AMP Content, compared to purified parathyroid hormone. Another important aspect is the study of the interaction of the Vitamin D3 metabolites on rat fetal bones, in vitro. This will be accomplished by measuring the release of 45 Ca from prelabeled rat fetal bones. Also, a simple bioassay method for 1,25-dihydroxyvitamin D3 in plasma will be devised using the rat fetal bone culture.